OpenMP: Add omp_get_initial_device/omp_get_num_devices builtins

By adding omp_get_initial_device and omp_get_num_devices builtins for
C, C++, and Fortran, the following can be achieved:
* By making them pure, multiple calls can be avoiding in some cases.
* Some comparisons can be optimized at compile time.

omp_get_initial_device will be converted to omp_get_num_devices for
consistency; note that OpenMP 6 also permits omp_initial_device (== -1)
as value.

If GCC has not been configure for offloading, either intrinsic will
leads to 0 - and on the offload side, -1 (= omp_initial_device) is
returned for omp_initial_device.

gcc/fortran/ChangeLog:

	* f95-lang.cc (ATTR_PURE_NOTHROW_LIST): Define.
	* trans-expr.cc (get_builtin_fn): Handle omp_get_num_devices
	and omp_get_intrinsic_device.
	* gfortran.h (gfc_option_t): Add disable_omp_... for them.
	* options.cc (gfc_handle_option): Handle them with
	-fno-builtin-.

gcc/ChangeLog:

	* gimple-fold.cc (gimple_fold_builtin_omp_get_initial_device,
	gimple_fold_builtin_omp_get_num_devices): New.
	(gimple_fold_builtin): Call them.
	* omp-builtins.def (BUILT_IN_OMP_GET_INITIAL_DEVICE): Add
	(BUILT_IN_OMP_GET_NUM_DEVICES): Make uservisible + pure.

libgomp/ChangeLog:

	* libgomp.texi (omp_get_num_devices, omp_get_intrinsic_device):
	Document builtin handling.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/omp_get_num_devices_initial_device-2.c: New test.
	* c-c++-common/gomp/omp_get_num_devices_initial_device.c: New test.
	* gfortran.dg/gomp/omp_get_num_devices_initial_device-2.f90: New test.
	* gfortran.dg/gomp/omp_get_num_devices_initial_device.f90: New test.

Co-authored-by: Sandra Loosemore <sloosemore@baylibre.com>

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